This invention relates to the field of external and implantable cardiac pacing systems incorporated into cardiac pacemakers or implantable cardioverter/defibrillators (ICDs), particularly atrial and ventricular synchronous pacing systems, wherein ventricular heart rate is stabilized or regularized during atrial tachyarrhythmias, particularly atrial fibrillation.
Episodes of atrial tachyarrhythmias, including atrial fibrillation (AF), are experienced by some heart patients. Although AF episodes may not be immediately life threatening, they may be associated with extreme symptoms, a reduced quality of life, and a reduced cardiac output. During AF episodes, the ventricular intervals may vary substantially from one ventricular cycle to the next if such patients have intact AV conduction. See R. J. Cohen et al., xe2x80x9cQuantitative Model For Ventricular Response During Atrial Fibrillationxe2x80x9d, IEEE Transactions on Biomedical Engineering Volume 30, pages 769-782 (1983). Ventricular rate irregularity is undesirable because it is uncomfortable for the patient, may increase susceptibility to ventricular tachyarrhythmias, and can decrease cardiac output.
Many patients are not symptomatic during AF. However, symptomatic AF patients are typically treated with drugs, e.g., amiodarone, to suppress the AF and maintain sinus rhythm. These xe2x80x9crhythm controlxe2x80x9d drug therapies are frequently ineffectual and/or have undesirable side effects. Alternatively, physicians prescribe drugs to reduce the ventricular heart rate (xe2x80x9crate controlxe2x80x9d). In some patients, it is not possible to achieve rate control due to ineffective drugs, side effects, contraindications, or lack of compliance by the patient. Physicians treat patients who fail rhythm control or rate control drug therapies with AV nodal ablation and pacemaker implantation. AV node ablation is undesirable because it causes irreversible destruction of the AV node, results in the patient being pacemaker dependent, and is associated with an increased risk for sudden cardiac death.
Many dual chamber pacing systems have been proposed or clinically used to provide bradycardia pacing and respond to AF in a variety of ways to reduce patient symptoms. For example, a variety of mode switching features have been disclosed which respond to an excessively rapid atrial rhythm by causing the pacing system to switch from an atrial synchronized pacing mode, such as DDD/DDDR, to a non-synchronized mode such as VVI/VVIR or DDI/DDIR. Such mode switching features are disclosed in U.S. Pat. No. 5,144,949, by Olson, U.S. Pat. No. 5,318,594, by Limousin et al., U.S. Pat. No. 4,944,298, by Sholder, U.S. Pat. No. 5,292,340, by Crosby et al., and U.S. Pat. No. 4,932,406 by Berkovits, all incorporated herein by reference in their entireties. In such pacing systems, the primary purpose of the mode switch is to prevent the pacing system from tracking a non-physiologic atrial rate. Unfortunately, these mode switching mechanisms do not address the problem of inappropriately high, irregular ventricular rhythms that often result during AF. Ventricular pacing is commonly inhibited because the intrinsic Ventricular rate of the AF patient is commonly faster than the rate of the pacemaker.
Various ventricular pacing regimens have been proposed to attempt to stabilize or regularize the ventricular heart rate and to avoid AV nodal ablation. Wittkampf et al., (xe2x80x9cRate Stabilization by Right Ventricular Pacing in Patients With Atrial Fibrillationxe2x80x9d, PACE, Vol. 9, November-December, 1986, Part II, pp. 1147-1153 and xe2x80x9cEffect Of Right Ventricular Pacing On Ventricular Rhythm During Atrial Fibrillationxe2x80x9d, JACC 11:539-545, 1988) proposed VVI pacing at a rate that results in a substantial percentage of depolarizations resulting from ventricular pacing, and a smaller percentage of depolarizations resulting from intrinsic conduction. The pacing rate was increased each time a ventricular sense event restarted the pacing escape interval and was decreased each time the pacing escape interval timed out and a ventricular pace was delivered. The articles state that this methodology provides stable ventricular rates in the presence of AF with only a moderate increase in overall ventriculiar heart rate.
Lau et al., xe2x80x9cA New Pacing Method for Rapid Regularization and Rate Control in Atrial Fibrillationxe2x80x9d, Am Journal of Cardiology, Vol. 65, pp. 1198-1203, May 15, 1990 attempted to stabilize the ventricular heart rate by triggering a ventricular pace after every ventricular sense. The average triggered pacing interval was about 230 ms. It is believed that this approach doesn""t effectively smooth out the ventricular rate and that it may be proarrhythmic in that the ventricular pace pulse may be delivered during the vulnerable period of ventricular repolarization.
Other DDD/DDDR pacing systems have been proposed that undergo mode switching in response to detected AF and attempt to stabilize the ventricular heart rate. Such pacing systems are disclosed in U.S. Pat. Nos. 5,480,413 and 5,591,215, both issued to Greenhut et al., and in U.S. Pat. No. 5,893,882 issued to Peterson et al., all incorporated herein by reference, which detect the presence of atrial tachyarrhythmia and switch from a dual chamber DDD/DDDR pacing mode or a VDD/VDDR pacing mode to a DDI/DDIR or VDI/VDIR pacing mode that effectively monitors the AF condition while pacing in the ventricle. In the ""413 and ""215 patents, the intrinsic ventricular heart rate irregularity is measured, and ventricular pacing rate is incremented and decremented based on the measured irregularity. However, pacing rate adjustments based on rate irregularity criteria require the time and energy consuming process of measuring the irregularity. The irregularity criterion also requires programming to set an appropriate target and an inappropriate irregularity criterion may result in ineffective pacing therapy. In the ""882 patent, upon mode switching in response to detected AF, the ventricular pacing rate is modulated based upon preceding ventricular intervals such that the current pacing interval is set equal to the preceding intrinsic or paced interval, with an increment if the preceding interval is less than the desired or target pacing interval, as typically will be the case, or with a decrement if the preceding interval is greater than the desired pacing interval. This approach may result in a sudden increase in pacing rate after a very fast intrinsic rate, which commonly occurs during AF.
An alternative approach that attempts to smooth the ventricular heart rate employing a physiologic rate or xe2x80x9cphys_ratexe2x80x9d and a xe2x80x9cflywheelxe2x80x9d rate is disclosed in U.S. Pat. No. 5,792,193 to Stoop, incorporated herein by reference, in the context of a DDD/DDDR mode switching pacing system. During AF, phys_rate xe2x80x9ctracksxe2x80x9d the average ventricular rate by incrementing a small amount when a ventricular sense occurs, and decrementing when a ventricular pace occurs. The escape pacing rate, or the flywheel rate, is set to be slightly slower than the phys_rate. If the flywheel interval associated with the flywheel rate times out, a ventricular pace is delivered. In this way, ventricular pacing occurs at a rate slightly slower than the mean rate, or phys_grate. However, depending on how quickly pacing rate is incremented after a ventricular sense, and how quickly pacing rate is decremented after a ventricular pace, this feature may result in pacing at an inappropriately fast rate. For example, if the ""193 feature increases pacing rate by 2 bpm after a ventricular sense and decreases by only 0.5 bpm after a ventricular pace, the feature may inappropriately pace at the upper pacing rate during an atrial flutter with 3:2 conduction.
What the art has not yet shown is a simple system that does not require complex computations of ventricular rate irregularity and stabilizes ventricular rate without increasing mean rate inappropriately.
The objective of the present invention is to regularize heart rate while avoiding pacing at inappropriately fast rates. The Ventricular Rate Regularization (VRR) feature of the present invention accomplishes this result by adjusting pacing rate according to the pattern of the most recent series of sensed or paced ventricular events. The invention may be embodied in an implantable or external cardiac pacemaker or ICD. Preferably, the invention is embodied in a device having rate-responsive pacing capabilities and operates to adjust the pacing rate of such device as it operates in DDIR, VDIR or VVIR mode.
The invention may be embodied in a dual chamber, rate responsive, pacing system operable in either the DDD or DDDR pacing mode, which, in case of detection of atrial tachyarrhythmia, automatically switches to a DDIR pacing mode to uncouple ventricular pacing from the atrial rate and stabilizes the ventricular rate by way of the VRR feature. The invention may also be embodied in a dual chamber, rate responsive, pacing system operable in the VDD or VDDR pacing mode, which, in case of detection of atrial tachyarrhythmia, automatically switches to a VDIR pacing mode and stabilizes the ventricular rate by way of the VRR feature. In these embodiments, the device operates as described in the above-cited Olson and Berkovits patents in DDIR and VDIR modes, respectively, but the escape interval of the device corresponding to the ventricular pacing rate is modulated by the VRR feature to provide a more regular rate. For patients whose atrial tachyarrhythmias are chronic, the pacing system may be programmed to the DDIR or VVIR modes with VRR operating continuously.
The VRR feature of the present invention regularizes ventricular rate by slightly elevating the pacing rate so that greater than 50% of ventricular events are paced and less than 50% of ventricular events are sensed. Ventricular pacing rate increase and decrease decisions are based on the pattern of the most recent sensed and paced ventricular events. A programmable rate limit (VRR URL) is provided to limit the maximum ventricular pacing rate that the VRR feature will achieve.
When the VRR feature is operative in the DDIR mode, the ventricular pacing rate defined by the present invention may also be influenced by the occurrence of an atrial pacing pulse delivered in the previous V-V interval if the most recent ventricular event was a sensed event (Vs). The VRR feature in this case does not increment the ventricular pacing rate if the Vs was due to intrinsic AV conduction after an atrial pace, because doing so could create positive feedback which would drive the ventricular pacing rate to the upper rate limit. Instead, the ventricular pacing rate is preferably decremented when an atrial pace is delivered in the preceding V-V sequence ended by a Vs.
In a preferred embodiment, the ventricular pacing rate is initialized at a physiologic pacing rate determined to meet the patient""s need for cardiac output, and the VRR feature makes rate increase and decrease decisions based on whether the previous two ventricular events were both sensed ventricular depolarizations or xe2x80x9cventricular sensesxe2x80x9d (Vs-Vs), both ventricular pacing pulses or xe2x80x9cventricular pacesxe2x80x9d (Vp-Vp), or a sequences of ventricular paces and senses (Vp-Vs or Vs-Vp). The VRR feature incrementally increases, maintains or decreases the ventricular pacing rate depending upon the ventricular event sequence to promote ventricular heart rate stability at the lowest possible ventricular pacing rate.
The VRR feature may also include a ventricular stability rate range (VRR Stable Range) operative in the DDIR, VDIR and VVIR pacing modes such that sensed ventricular events in this range will not increment the ventricular pacing rate when they are considered to be stable. The current V-V cycle length terminated by a ventricular sensed event is measured. No increments, and preferably no changes whatsoever are made to the ventricular pacing rate if the current V-V cycle length is within the stable range, as calculated based upon one or more preceding V-V cycle length measurements.
Other features and attributes of the invention shall become apparent from the following description of the invention. The rhythms that are recognized and/or responded to in use of the VRR feature may comprise any of the supraventricular arrhythmia types, e.g., supra-ventricular tachycardias, atrial fibrillation and atrial flutter, but the invention is especially benefical in the case of patients suffering from atrial fibrillation.